"Freedom. It is an ideal for which I am prepared to die," says 44-year-old Nelson Mandela just before he is sentenced to life in prison for sabotage and conspiracy to overthrow the South African government in Justin Chadwick's reverent but rousing Mandela: Long Walk to Freedom. The facts of Mandela's influential work to end apartheid, from his early days as a lawyer in Johannesburg to his rise in the African National Congress, as well as his 27-year imprisonment and ultimate release and election as South African president, are well-known, and Chadwick's straightforward retelling of his story doesn't offer any new insights into the man. But that's OK. Mandela's trials and triumphs, personal and political, are more than enough to keep this crowded but never overstuffed biopic clicking. Idris Elba, perhaps best known in the U.S. for his riveting work on HBO's The Wire and BBC America's Luther, vividly embodies Mandela's pride and passion -- and flaws. While the film is certainly respectful toward Mandela, it isn't hagiographic. Early on, we see Mandela's flirtations with women, his ill treatment of his first wife, and his decision to jettison his nonviolent ways and authorize bombings on government targets. In the early stages of the movie that trace his youthful zeal and rise to power in the ANC, Elba imbues Mandela with undeniable charisma and unbridled passion. It is clear why so many listened to his speeches and followed his lead. But as the film progresses and he ages in prison, Elba matures in the role and doesn't need words to convey Mandela's authority, unquestioned character, and gentle spirit. Just as Daniel Day-Lewis inhabited Abraham Lincoln and made viewers forget that they were watching an actor in a role, Elba achieves a similar effect. He becomes Mandela. It's an understated, convincing, and powerful performance. And as good as Elba is, he is matched by Naomie Harris as Winnie Mandela in a harrowing, towering tour de force. Mandela is immediately smitten with the winsome Winnie, calling her "the most beautiful girl I have ever seen." They soon marry and have children, but their life is anything but ideal. When the two are eventually separated due to Nelson's imprisonment, Winnie continues to live in her husband's shadow and is subjected to threats, torture, and 18 months in solitary confinement. The person who emerges from that dank prison cell is no longer winsome but angry, determined, and absolutely fearless. She is a woman to be reckoned with, a fierce force of nature. She wants the white oppressors to pay, while Nelson forgives them and seeks peace. Upon his release, their conflicting outlooks lead to their separation. Chadwick doesn't shy away from showing the harsh realities of apartheid or the cruel treatment Mandela and Winnie received in prison, but his approach is restrained. His camera doesn't linger over dead bodies or focus at length on beatings or torture. This is a story hurtling towards a triumphant ending, where grace and peace overpower evil and violence. If there is a negative to Chadwick's film, it's that it is a bit overlong and takes a half hour or so to really get going. But that is a minor quibble. Mandela: Long Walk to Freedom may be long but it is never boring, and the journey it takes is essential viewing for anyone interested in Mandela, the history of apartheid, South Africa, or social justice. Or great acting.